ITD Method: A new lens to investigate the planetary ionosphere

“This article presents a recent advancement in planetary science that redefines how we interpret radio signals from space. Researchers developed a novel algorithm, the Integrated TEC Decomposition (ITD) method, that corrects for plasma asymmetry in the Venusian ionosphere, a challenge that has long been due in radio occultation analysis. By eliminating assumptions of spherical symmetry, this technique enhances the precision in the retrieval of electron density profiles. The implications stretch far beyond Venus, promising improved insights into other planetary bodies, mainly for giant planets like Jupiter and Saturn.“

Introduction

Did you know that our understanding of a planet’s atmosphere can be shaped by how a simple radio signal bends as it travels through space?

For decades, scientists have used radio occultation, a method where spacecraft transmit radio waves that pass through planetary atmospheres, and the signal suffers bending due to the thick atmospheric density before reaching the receiver. This technique offers an elegant way to explore alien worlds without landing on them. But there’s a catch: the conventional method assumes that these atmospheric layers are perfectly symmetrical, but reality is not.

Recent research led by the team at the University of Tokyo has challenged this long-standing assumption and introduced a new method that better reflects the dynamic, asymmetric nature of planetary ionospheres. The impact of this work could help better understand the aeronomy of the planetary ionosphere, which supports the future planetary mission.

Motivation

Traditional radio occultation techniques rely on a simplified assumption: that the ionosphere surrounding a planet is spherically symmetric. This assumption is mathematically convenient, helps to divide the atmosphere into concentric spherical shells, and estimates the properties of each shell one by one to get the complete information about the atmosphere. However, for planets like Venus, where the signal traverses a long distance before emerging out through the atmospheric medium, hence change in medium along the ray path due to differential heating by the Sun and other atmospheric dynamics is significant. This may lead to overestimation/ underestimation of the derived atmospheric parameters from the true value.

As a result, this symmetry bias can significantly deviate from the actual measurements, particularly at lower altitudes where the signal path increases due to. This poses a major problem for researchers attempting to understand how solar radiation, magnetic fields, or even gravitational waves from below influence plasma behavior in the lower region of the planetary ionospheres.

So the burning question arises: Can we measure planetary ionospheres more accurately by correcting for real-world asymmetries?

Research Methodology

To answer that question, the researchers devised a new retrieval algorithm called the Integrated TEC Decomposition (ITD) method. TEC, or Total Electron Content, refers to the number of electrons a radio signal encounters along its path through a plasma-rich environment in a unit circular area.

Instead of assuming symmetry and applying the standard Abel transformation, the ITD method slices the ionosphere into thin spherical shells and calculates local electron densities by directly analyzing phase shifts in the radio signal. It accounts for changing solar zenith angles (SZA), the angle between the Sun and the observed location, which affects electron production in the ionosphere.

Figure: Schematic diagram to represent the basic algorithm of the ITD method. Here Star in yellow color mimics the position of the Sun.

By combining data from two space missions, Venus Express (VEX) and Akatsuki, the method was validated for both dual-frequency and single-frequency radio signals. The study focused particularly on the 90–180 km altitude range, where plasma asymmetry is most pronounced due to solar-driven photochemistry.

Results

The new ITD method revealed that conventional techniques overestimate electron densities by 2–5% near the ionospheric peak and by up to a staggering 800% at lower altitudes. These errors arise due to uncorrected plasma asymmetries influenced by SZA variations.

One key takeaway: even small angular variations in sunlight can impact the results on planetary ionospheres, especially at low altitudes. By accurately correcting for these variations, the ITD approach is very simple and yields a much clearer picture of the Venusian ionosphere, especially near the planet’s solar terminator.

Conclusion and Future Scope

This breakthrough goes far beyond Venus. The proposed method can be more useful for giant planets, such as Jupiter, Saturn, and planets where spherical symmetry is not a big challenge, like the Moon and even Mercury. It has particular relevance for upcoming missions like India’s Venus Orbiter Mission (VOM) and ESA’s EnVISION missions, etc.

But why does this matter to the rest of us?

The same radio occultation principles used here are also relevant for satellite navigation, weather forecasting, and space traffic monitoring. Improving the accuracy of plasma measurements helps scientists understand space weather impacts, which have implications for our satellites, communication systems.

By offering a more realistic retrieval algorithm and better understanding of how plasma behaves in a complex space environment, this research opens new frontiers, not just in planetary science but also in how we interpret and interact with space from Earth.

Reference

Tripathi, K.R., Imamura, T., & Choudhary, R.K. (2025). A novel technique for plasma asymmetry correction in radio occultation profiling, Icarus, 441, 116670. https://doi.org/10.1016/j.icarus.2025.116670